Adding new libc configure options

There are a number of configure options which can be used to configure the libc build. The config system is driven by a set of hierarchical JSON files. At the top of the hierarchy is a JSON file by name config.json in the config directory. This JSON file lists the libc options which affect all platforms. The default value for the option and a short description about it listed against each option. For example:

{
  "printf": {
    "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
      "value": false,
      "doc": "Disable printing floating point values in printf and friends."
    }
  }
}

The above config indicates that the option LIBC_CONF_PRINTF_DISABLE_FLOAT has a value of false. A platform, say the baremetal platform, can choose to override this value in its config.json file in the config/baremetal directory with the following contents:

{
  "printf": {
    "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
      "value": true
    }
  }
}

Here, the config for the baremetal platform overrides the common false value of the LIBC_CONF_PRINTF_DISABLE_FLOAT with the true value.

Config JSON format

Named tags

As can be noted from the above examples, config.json files contains a top-level dictionary. The keys of this dictionary are the names of grouping-tags. A grouping-tag is nothing but a named tag to refer to a related group of libc options. In the above example, a tag named printf is used to group all libc options which affect the behavior of printf and friends.

Tag values

The value corresponding to each grouping tag is also a dictionary called the option-dictionary. The keys of the option-dictionary are the names of the libc options belonging to that grouping tag. For the printf tag in the above example, the option-dictionary is:

{
  "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
    "value": false,
    "doc":
  }
}

The value corresponding to an option key in the option-dictionary is another dictionary with two keys: "value" and "doc". The "value" key has the value of the option listed against it, and the "doc" key has a short description of the option listed against it. Note that only the main config file config/config.json includes the "doc" key. Options which are of ON/OFF kind take boolean values true/false. Other types of options can take an integral or string value as suitable for that option. In the above option-dictionary, the option-key LIBC_CONF_PRINTF_DISABLE_FLOAT is of boolean type with value true.

Option name format

The option names, or the keys of a option-dictionary, have the following format:

LIBC_CONF_<UPPER_CASE_TAG_NAME>_<ACTION_INDICATING_THE_INTENDED_SEMANTICS>

The option name used in the above examples, LIBC_CONF_PRINTF_DISABLE_FLOAT to disable printing of floating point numbers, follows this format: It has the prefix LIBC_CONF_, followed by the grouping-tag name PRINTF in upper case, followed by the action to disable floating point number printing LIBC_CONF_PRINTF_DISABLE_FLOAT.

Mechanics of config application

Config reading

At libc config time, three different config.json files are read in the following order:

1. config/config.json

2. config/<platform or OS>/config.json if present.

3. config/<platform or OS>/<target arch>/config.json if present.

Each successive config.json file overrides the option values set by previously read config.json files. Likewise, a similarly named command line option to the cmake command will override the option values specified in all or any of these config.json files. That is, users will be able to override the config options from the command line.

Config application

Local to the directory where an option group is relevant, suitable build logic should convert the CMake config options to appropriate compiler and/or linker flags. Those compile/link flags can be used in listing the affected targets as follows:

add_object_library(
 ...
 COMPILE_OPTIONS
   ${common_printf_compile_options}
   ... # Other compile options affecting this target irrespective of the
       # libc config options
)

Note that the above scheme is only an example and not a prescription. Developers should employ a scheme appropriate to the option being added.

Automatic doc update

The CMake configure step automatically generates the user document doc/configure.rst, which contains user information about the libc configure options, using the information in the main config/config.json file. An update to config/config.json will trigger reconfiguration by CMake, which in turn will regenerate the documentation in doc/configure.rst.